A typical data storage system includes one or more rack systems, each rack system having a plurality of storage devices or enclosures. A common topology in which the enclosures are connected is a loop. Communication signals traverse the loop uni-directionally and pass from enclosure to enclosure in a daisy-chain fashion. Enclosures receiving communication signals targeted to another enclosure forward those signals along the loop.
As communication signals traverse various media comprising the loop, such as copper cables, fiber optics, and printed circuit boards, the “data eye” of the signals, i.e., the region of valid data, tends to degrade due to attenuation and jitter (i.e., variations in the frequency or phase of the signals, due to varying time delays incurred on the media). Unless an enclosure compensates for the attenuation and jitter, the data eye can degrade to the point where the data value represented by the signal becomes erroneously interpreted (a bit error). Accordingly, industry has developed enclosures that amplify the communication signals and reduce jitter, relegating the task of improving or eliminating jitter to the disk modules within the enclosure.
Although this technique alleviates the problem of jitter, it also requires every enclosure within a data storage system to have at least one or, more often, two disk modules (one disk module for each direction in which the loop passes through the enclosure). In practice, users find even two disk modules inadequate because having only one disk module in each loop direction renders the storage system vulnerable to the failure of a single disk. Typically, storage systems permit hot swapping of disk modules, but hot-swapping the only disk module in a loop direction through the enclosure eliminated the only means for removing jitter. Thus, because of a single disk failing, the entire loop fails. To avoid this vulnerability, a user would necessarily install a full complement of disk modules to implement redundancy, and thus allow for hot-swapping. Consequently, this situation prevents users from configuring their storage systems with diskless enclosures, which is a need felt by users who want to configure their storage systems with expansion capabilities without having to fully populate the enclosures with disk modules, and still be able to test the loop connectivity and functionality of these enclosures.